This invention relates to lithographic plates. More particularly, it relates to a colored positive-acting, radiation-sensitive composition, useful in the preparation of lithographic printing plates, whose color is extinguished or greatly reduced in intensity upon exposure to radiation thereby providing great contrast between image and non-image areas on said plates prior to development, comprising at least one acid-free resin, at least one acid-sensitive dyestuff and at least one positive-acting component which generates an acidic product upon irradiation. It also relates to a positive-acting, radiation-sensitive element comprising a support material at least one surface of which has been coated with the above composition. It also relates to a lithographic printing plate of high contrast between the deeply colored image areas before development, prepared from said element, wherein the support material is lithographically suitable by imagewise exposure. Said image areas are highly stable to moisture and alkali developers and more highly colored than the revealed substrate surface after development.
Depending upon the nature of the radiation-sensitive coating employed a lithographic printing plate may reproduce the image to which it is exposed, in which case it is termed a positive-acting plate, or produce an image complementary to the one to which it is exposed in which case it is termed a negative-acting plate.
A positive-working printing plate is then, generally, one in which the non-image area is the portion of the radiation-sensitive composition exposed to radiation and thereby rendered more developer soluble than the unexposed portions which are inherently, or are chemically hardened and rendered, oleophilic and, therefore, ink receptive.
It is highly desirable, in the art of preparing positive-working lithographic printing plates, to prepare an image which is easily visible to the plate maker immediately after exposure to radiation, but before development, so that the skilled worker can see and compose the letters, numbers, and symbols on the plate after each exposure and make such alterations and modifications as may be necessary. This would not be possible if the image were not visible immediately after exposure. Such visibility depends upon differences in the intensities and/or colors of room light reflected from the exposed and unexposed areas of the plate. It is well known in the art to obtain such differentials in positive-working plates where the exposed, or non-image, areas of the plate become darker upon irradiation resulting in a greater image visibility as compared to the unexposed, image areas which retain the original color of the composition as prepared by the manufacturer. However, in general, those differentials are not the most desirable since the background or non-image areas are in effect more intense in color than the unexposed areas which will form the actual image to be printed. It is, therefore, more desirable to obtain a visibility differential between the background and the image wherein the characteristics are reversed; that is, where the image is rendered more intense in color than the exposed non-image areas. This has the additional advantage of providing for great contrast between the image and non-image areas of the subsequently developed plate.
One attempt at producing a positive-working lithographic printing plate wherein the exposed, non-image areas are rendered lighter than the unexposed, originally colored, image areas, thereby creating the desired difference in visibility is described in U.S. Pat. No. 3,969,118. According to that method, a positive-working, radiation-sensitive ester of naphthoquinone-(1,2-)-diazide-(2)-5-sulfonic acid is blended with an organic dyestuff and a halide of naphthoquinone-(1,2)-diazide-(2)-4-sulfonic acid.
However, such a system suffers from inter alia, being very sensitive to moisture and alkalies, e.g., developers. Thus, while the color differential between the image and non-image areas rises as the concentration of the sulfonyl halide increases, the hardness of the printing surface decreases and it is more easily attacked by the alkaline developers. The resultant plate, consequently, yields fewer prints than a lower contrast plate.
If, however, the sulfonyl chloride is eliminated it is found that the composition, throughout the coating, is partially decolorized prior to exposure thereby yielding a product of decreased contrast between the exposed and unexposed areas thereof. It is believed that this result is due to the presence of acid in the resin components of the composition which has remained from the process of preparing said resins.
It has now been found that the above disadvantages may be overcome by the use of the composition according to the instant invention, in the preparation of the lithographic printing plates, which results in exposed, but undeveloped plates, of more stable and intense color contrasts and, additionally, longer running and more stable developed plates.